Human and animal blood specimens are frequently analyzed to determine the concentrations of components useful in diagnosis and preventive medicine. To perform these analyses, whole blood is separated into the components of plasma and particulates such as blood cells, or coagulated or clotted blood is separated into serum and the solids. Such separations are preformed by centrifuging. In centrifuging, the serum or plasma is lighter in weight and collects radially closer to the spin axis of the centrifuged container. A thixotropic gel, which has a density between the density of the blood cells and the density of the plasma, is generally used to maintain the separation between the blood cells and the plasma.
A centrifuge system employing a disc-shaped cassette or rotor is known, which can accomplish the desired separation rapidly. U.S. Pat. Nos. 4,846,974 and 4,981,585 disclose a suitable centrifuge system of this type.